Intrinsically safe switching system



March 7, 1961 A. G. LE LACHEUR INTRINSICALLY SAFE SWITCHING SYSTEM Filed Sept. 24, 1957 INVENTOR an {1414 a 7,

Unite My invention relates to an intrinsically safe electrical switching system for explosive atmospheres even though there is atmospheric communication between the contacts of the switching device, its associated wiring, and the hazardous gaseous mixture involved.

The present conventional method of switching in hazardous atmospheres depends for its operational safety on so-called explosion proof enclosure equipment. This explosion proof equipment is intended to isolate the switch and its associated wiring from the surrounding explosive gas/air or vapor/air mixtures. Individuals acquainted with this method recognize certain inherent Weaknesses that will render it unsafe and cannot be corrected mechanically.

In the operation of switches in hazardous atmospheres there are always present two elements that contribute to an explosion, the electrical spark at the switch contacts when the current is interrupted, and the gaseous mixture involved. If either of these two elements can be eliminated, or rendered incapable of contributing to an explosive condition, the switching operation will be non hazardous.

The principal object of my invention is to provide a new, safer and potentially less expensive switching device for explosive atmospheres, even though the contacts of the switch are exposed and have atmospheric communication with the surrounding hazardous vapor/air mixtures. This switching system is based on the fact that a measurable and known amount of electrical energy is needed to ignite various mixtures of explosive gases. Essentially a remote control device, it employs a low, safe level of electrical energy to control circuits or devices requiring higher electrical levels. This switching system.which is comprised of an actuating switch, conductively connected to a remote control circuit relay, is designed so the electrical spark energy level supplied by the remote control circuit to the actuating switch in the explosive atmosphere, is below the minimum required to cause ignition to all generally known gas/air or vapor/air mixtures even under ideal potential explosive conditions.

A further object of my invention is to provide an intrinsically safe method of switching for explosive atmospheres even under abnormal conditions. Such abnormal conditions include accidental damage to any part of the equipment or wiring, insulation or other failure of electrical components, application of overvoltage, adjustrnent and maintenance operations, and other similar conditions.

Figure 1 of the drawing shows a simple installation of the present invention.

The schematic diagram of Figure 2 shows a control circuitand switching means for actuating the control circuit. The control circuit being comprised of impedance means for reducing and limiting a source voltage to an intrinsically safe actuating energy incapable of igniting upon interruption any of the generally known 2,974,261 Patented Mar. 7, 1961 ice Class 1 Hazardous Atmospheres, means for supplying this less minimum electrical spark ignition energy to switching means capable of having atmospheric communication with an explosive atmosphere and means for raising the said less than minimum electrical spark ignition energy to higher electrical energy levels. As various type components have failure characteristics not common to each other, Figure 2 shows, based on these characteristics, an arrangement of components in series that limit the said actuating energy to the said less than minimum electrical spark ignition energy even under abnormal conditions or likely conditions of failure that could occur in the control circuit.

Figure 3 of the drawing shows an alternative method of reducing and limiting a source voltage to an intrinsically safe actuating energy for explosive atmospheres.

Referring to the drawing, Figure l is a block diagram showing an actual installation. The enclosure 1, represents an area containing any of the Class 1 Hazardous Atmospheres. The control circuit 2, is located in a non hazardous atmosphere and is prevented from having atmospheric communication with the hazardous area by the vapor seal 3. Control circuit 2 includes voltage dropping means for providing a non-igniting actuating electrical spark energy for switch 4 and amplifying means to raise this low level energy to higher electrical energy levels. Switch 4 and that part of the connecting conductor 5 located in the hazardous area can have atmospheric communication with the surrounding explosive vapor/ air mixtures. 49 is the voltage supply connecting to an alternating current source. 50 is the load terminals. The closing of switch 4 actuates the control circuit 2 in such a way as to cause a relay to respond to the switch operation.

Figure 2 of the drawing shows a control circuit comprised of an impedance bridge circuit and amplifying means. The elements in the bridge circuit are of optimum values of resistance and reactance that will result in lowering the bridge input voltage to an energy incapable of igniting any known gaseous mixture when applied to the actuating switch 4 in the hazardous atmosphere.

The connecting control conductor is a two wire, grounded metallic sheath cable. One of the wires of control conductor 5 connects to the junction of bridge elements 6, 8, and 10, with no physical connection at the other end of the wire. The remaining wire of the control conductor connects one terminal of switch 4 to the junction of bridge elements 7, 9, and 11. The remaining terminal of the switch connects to ground, either conductively or capacitively. The two wire control conductor cable having its sheath connected to the common control circuit ground becomes the electrical equivalent of the two grounded capacitors of the bridge circuit thereby extending the electrical function of these capacitive bridge elements the entire length of the control conductor cable to the switch location in the hazardous atmosphere.

The bridge elements 6, 7, 8, 9, 10, and 11 are enclosed in a metal container that is grounded. These elements can be all, either resistive or capacitive, but for good stability and eliminating phase shift due to conductor 5 adding capacity to the bridge it is preferable that elements 8 and 9 be capacitive. Coupling capacitor 14 serves as a blocking condenser and its only purpose is to prevent any DC. voltage from appearing at the actuating switch contacts in event there should occur an internal shorting of the pentode tube 15 elements to the control grid.

The primary of transformer 13 is connected to an A.C. supply source. The secondary winding has a grounded center tap and supplies the bridge circuit with a voltage.

Potentiometer 12 is shunted across the secondary wind-' ing with potentiometer center tap grounded. This poteniometer is used to balance the bridge circuit by varying the input bridge voltage either side to ground. Balance means being located on the low impedance side of the bridge circuit are less critical in adjusting than if they were in the bridge circuit proper. The wire of the control conductor that has physical connection at one end only acts as a capacitor and practically eliminates any rebalancing of the bridge circuit that otherwise would be necessary when changes occurred in control distances, temperature, and humidity.

The electron tube circuit consists of a pentode tube 15 amplifier stage capacitively coupled to the first half of the twin triode 24 which acts as another amplifier stage; the output of the second stage, in turn, is R-C coupled through dry rectifier 31 to the grid of the other half of the twin triode 24. The controlled relay 36 is in series with the plate of the second half of tube 24 and the DC. voltage supply provided by dry rectifier 39 and associated resistors and capacitor in the power transformer 37 secondary circuit. The primary circuit of the power transformer is connected to an A.C. supply source. Consequently, when the output stage of tube 24 conducts, relay 36 is energized.

Under balanced bridge conditions, there is no output voltage from the bridge to be applied to the grid of the pentode tube 15. Therefore, there is no signal to be amplified, rectified and applied to the grid of the power section of tube 24. Because of cathode bias resistor 35 connected to the DC. supply, the output stage of tube 24 is cut off and relay 36 is de-energized.

Under balanced bridge conditions, it is upset whenever switch 4' is closed because this has the effect of shorting out capacitor 8 and grounding the respective bridge input junction. Consequently, a small potential (fraction of a volt) appears across resistor 16 which is amplified, rectified, and applied to the grid of the output stage of tube 24. This applied grid potential, being of positive polarity, overcomes the cathode bias and the stage conduts heavily enough to energize relay 36.

Figure 3 of the drawing shows another means of providing a non-igniting actuating energy for switch 4 located in an explosive atmosphere. The voltage dropping impedance circuit consists of resistor 44 and capacitor 45 which are of such optimum values that the voltage provided by the secondary winding of transformer 43 and supplied to switch 4 by the conductor 48, will be reduced to a low level electrical actuating energy incapable of causing ignition to any of the generally known hazardous atmospheres upon interruption by the opening or closing of the contacts of switch 4.

The primary transformer 43 is supplied with an A.C. voltage. Capacitor 46 serves as a blocking condenser. 47 is the junction point at which the voltage dropping impedance circuit can be connected to the amplifier circuit in Figure 2.

With switch 4 contacts open, the relay 36 is energized. Closing of the switch to ground shorts out capacitor 45, thus removing the small voltage that was being applied to the control grid of pentode tube 15. With the signal removed there is no amplified voltage to be rectified and applied to the grid of the second half of the twin triode 24. This section of the twin triode ceases to conduct and relay 36 becomes de-energized. The conductor 48 is a single wire with a grounded metallic shield. Capacitor 45 can be replaced by a resistor of optimum resistance.

Although Figure 3 shows a circuit that will provide a less than minimum electrical spark ignition energy for actuating, the circuit of Figure 2 is preferable from the point of reliability and stability under adverse operating conditions.

It will be recognized that circuit details shown herein for providing an electrical actuating energy that is insutficient to ignite any of the generally known explosive gas/air, and vapor/air mixtures may be varied without departure from the principles of this invention. Accordingly, it should be realized that this invention is to be considered as limited only to the extent required by the claims appended hereto.

What I claim is:

1. In an intrinsically safe switching system for hazardous locations the combination of an explosive atmosphere; general purpose switching means capable of having atmospheric communication with the said explosive atmosphere; an intrinsically safe actuating energy incapable of igniting the said explosive atmosphere when interrupted by the said switching means; a source of alternating current; a remote control apparatus and vapor seal means preventing travel of the said explosive atmosphere from the hazardous location to the said remote control apparatus; said remote control apparatus being comprised of means for reducing and limiting the said source current to the said intrinsically safe actuating energy, means for supplying the said intrinsically safe actuating energy to the said switching means, a change responsive amplifying circuit adapted to raise the said intrinsically safe actuating energy to higher electrical energy levels, impedance means supplying the said actuating energy to the input of the said amplifying circuit, said amplifier circuit responsive to the change in potential of the said actuating energy when the said switching means are 0perated; said mentioned means for reducing and limiting the said source current to the said intrinsically safe actuating energy being a combination of elements of optimum values of reactance and resistance or reactance or resistance that reduce and limit the said source current to the said intrinsically safe actuating energy; said mentioned elements being arranged in series and having electrical failure characteristics not common to each other with said arrangment being such that electric failure of any one of the said mentioned elements will not endanger the intrinsically safe operation of the said switching means in the said explosive atmosphere.

2. In an intrinsically safe switching system for hazardous locations the combination of an explosive atmosphere; switching means having atmospheric communication with the said explosive atmosphere; a remote control apparatus comprised of a normally balanced bridge circuit system having an input set of junctions and an output set of junctions and including-six impedance elements, two of which are capacitive and grounded; a grounded center tapped source of transformed alternating current; means for supplying the said current source through two of the said six impedance elements; adjustable bridge circuit system balancing means to ground being connected across the said current source; said set of input junctions being connected to a two conductor shielded control cable; said mentioned cable shield, said mentioned capacitive elements of the said bridge circuit system and said mentioned current source center tap being connected to a common ground; said switching means in hazardous location being connected across one of the two said control conductors to said mentioned common ground, the remaining conductor of the two said control conductors being terminated at the said switching means location without any electrical connection attached thereto; said shielded control cable being the electrical equivalent in function as the said grounded capacitive elements of the said bridge circuit system thereby extending the electrical function of the said capacitive bridge circuit elements the entire length of the said control cable to the said mentioned switching means location; said control cable maintaining the said bridge circuit system in balance without readjustment of the said balancing means in spite of variations in control distances, temperature, and humidity; said bridge circuit system elements being a combination of optimum values of reactance or resistance or reactance and resistance that reduce and limit the said source current to a less than minimum electrical spark ignition energy at the said input set of junctions and the said switching means in the hazardous location; the aforesaid elements having electrical failure characteristics not common to each other of either opening or shorting when tailing electrically, with said elements electrical relationship being such that electrical failure of any one of the said elements will not endanger the intrinsically safe operation of the said switching means in the said explosive atmosphere; amplifying means having a relay in its load circuit and adapted to operate the said relay when the said bridge circuit system is unbalanced by the operation of the said switching means; said switching operation being performed in the presence of explosive atmospheres without ignition or" the said explosive atmospheres being in attendance; means for preventing the said explosive atmosphere from traveling from the said hazardous location to the said remote control apparatus.

3. In an intrinsically safe switching system for hazardous locations the combination of an explosive atmosphere; general purpose switching means having atmospheric communication with the said explosive atmosphere and being supplied with a less than minimum electrical spark ignition actuating energy; a grounded source of alternating current; a remote control apparahis and means for preventing the said explosive atmosphere from communicating with the said remote control apparatus; said remote control apparatus being comprised of an energy limiting impedance circuit being connected across the said grounded source of alternating current and having an output junction, said switching means being connected across the said output junction and the grounded junction of the said current source, amplifying means having an impedance connected to the said output junction and adapted to raise the said less than minimum electrical spark ignition actuating energy to higher electrical energy levels, said amplifying means being responsive to change of potential at the said output junction when the said switching means are operated; the elements of the said energy limiting impedance circuit being a combination of optimum values of reactance or resistance or reactance and resistance that reduce and limit the said source current to the said less than minimum electrical spark ignition actuating energy;

said mentioned impedance elements being arranged in series and having electrical failure characteristics of either opening or shorting when failing with said arrangement being such that failure of any of the said elements will not endanger the intrinsically safe operation of the said switching means in the said explosive atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 1,602,993 Tinsley et al. Oct. 12, 1926 1,688,038 Curtis Oct. 16, 1928 2,089,997 Farnham Aug. 17, 1937 2,624,813 Guifirida Jan. 6, 1953 FOREIGN PATENTS 521,807 Great Britain May 31, 1940 549,679 Great Britain Dec. 2, 1942 

